Abstract
Currently, there are no effective herbicides registered for the control of hoary cress (Cardaria draba) in alfalfa (Medicago sativa) or alfalfa/grass mixtures. Due to this lack of control options, field experiments were conducted near St. Ignatius, MT, from 1995 through 1997 to determine the optimum imazethapyr rate and adjuvant combination for hoary cress control in alfalfa/orchardgrass (Dactylis glomerata) pastures. Imazethapyr was applied in the spring to rosette stage hoary cress plants at 33, 67, and 100 g ai/ha with either a nonionic surfactant or methylated seed oil alone or combined with 28% urea ammonium nitrate (UAN) liquid fertilizer. Hoary cress control was poorest when imazethapyr was applied at 33 g/ha, yet provided a minimum of 80% control. Imazethapyr applied at 67 and 100 g/ha provided greater than 90% control during the year of application regardless of surfactant or UAN additions. None of the treatments controlled hoary cress the year following application. Alfalfa yield was unaffected by imazethapyr applications. First-cutting orchardgrass yield decreased as imazethapyr rate increased, but total seasonal forage yield was unaffected.
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